Refactor editor selection code to be less hardcoded.

This means that individual wikis can set their editor preferences.

Note that now, editors MAY declare an isSupported() function. If this function returns false,
the next editor in the preference list will be loaded instead.

Change-Id: I23abeefafdbc79ac9107334ad8fb96ca4935ecb4
Merge "Refactor editor selection code to be less hardcoded."